1. Field of the Invention
This invention relates to a method for controlling the engagement of a probe within a scanning probe microscope, and, more particularly, to controlling a segmented piezoelectric actuator so that a first segment of the actuator responds to changes in the distance between the probe tip and the sample surface which occur rapidly as the sample is traversed by the probe during the scanning process, while a second segment of the actuator responds to changes in the distance between the probe tip and the sample surface which occur slowly as the sample surface is traversed by the probe during the scanning process.
2. Background Information
Conventional scanning probe microscopes employ a single piezoelectric actuator to move the probe in the engagement direction, which is often called the vertical or z-direction. Such devices often encounter a problem associated with an inability to obtain a very high digital bit resolution of the actuator movement together with a suitably large dynamic range of movement. For a particular piezoelectric material, the maximum dynamic range which can be achieved is determined by the piezo constant, C, expressed in Angstroms/Volt, and the maximum operational voltage, V.sub.max. The bit resolution, in Angstroms/bit is given by the following expression: ##EQU1## In this expression, R is the bit resolution, and N is the bit count. For example, if the digital signal generated within a computing device is fed through a 16-bit digital to analog converter in order to generate the analog signal needed to drive the actuator, the bit count is 65,535.
What is needed is a way to obtain high resolution without sacrificing the vertical (engagement) probe travel needed in many applications.
3. Description of the Prior Art
The patent art includes a number of patents, such as U.S. Pat. Nos. 4,343,993 and 4,724,318, describing scanning probe microscope technologies in which the present invention can be applied.
U.S. Pat. No. 4,343,993 describes a scanning tunneling microscope, in which a very sharp metal tip is raster-scanned across a surface to be inspected at a distance so small that the electron clouds of the atoms at the apex of the tip and on the surface area closest to the tip gently touch. A so-called tunnel current then flows across the gap, provided a potential difference exists between the tip and the surface. Since, this tunnel current is exponentially dependent on the distance between the tip and the surface, a correction signal is generated, based on deviations from a predetermined value occurring as the tip is scanned across the surface of the probe. The correction signal is used to control the tunnel distance so as to maximize the correction signal, and to be plotted versus a position signal derived from the physical position of the tip over the surface being inspected. This technique permits a resolution down to an atomic scale, so that individual atoms on the surface can be made visible.
U.S. Pat. No. 4,724,318 describes an atomic force microscope, in which a sharp point is brought so close to the surface of a sample to be investigated that the forces occurring between the atoms as the apex of the point and those at the surface cause a spring-like cantilever to deflect. The cantilever forms one electrode of a tunneling microscope, the other electrode being a sharp tip. The deflection of the cantilever provokes a variation of the tunnel current, and that variation is used to generate a correction signal which can be employed to control the distance between the point and the sample, in order, for example, the force between them constant as the point is scanned across the surface of the sample by means of an xyz-drive, with the sample being driven in a raster scan in the xy-plane. In certain modes of operation, either the sample or the cantilever may be excited to oscillate in the z-direction. If the oscillation is at the resonance frequency of the cantilever, the resolution is enhanced. Using this method, a topographical image of a sample surface having a resolution better than 100 nanometers may be obtained by employing the following steps: A sharp point which is fixed to one end of a spring-like cantilever is brought so close to the surface of the sample to be inspected that the forces occurring between the point and the sample surface are larger than 10.sup.-20 Newton, so that the resulting force deflects the cantilever. The deflection of the cantilever is detected by means of a tunnel tip disposed adjacent the cantilever. The tunnel current then flowing across the gap between the cantilever and tunnel tip is maintained at a constant value by using any detected variations of the tunnel current to generate a corrections signal. The correction signal is used, among other things, to maintain the point-to-sample distance constant.
While the abovementioned U.S. Patents provide descriptions of methods for using a scanning probe to develop data describing the surface of a test sample, the use of dual actuators to derive the benefits of relatively large movements with a slow actuator and of speed and accuracy with a fast actuator is not described. Thus, what is needed is a dual actuator system including a suitable control scheme for dividing required movements between the two actuators.
Other examples from the Patent Art, such as U.S. Pat. No. 5,414,690 and Japanese patent Kokai No. 4-318404, describe methods to move a scanning probe attached to a fine movement actuator which is, in turn, fastened to a course movement actuator.
U.S. Pat. No. 5,414,690 describes a method for moving a probe in a direction perpendicular to the surface of a sample by means of a fine movement section attached to the probe and a course movement section attached to the fine movement section. The fine movement section uses, for example, a piezo-electric actuator, while the course movement section is movable over a wide range by the use, for example, of a stepping motor.
Japanese patent Kokai No. 4-318404 describes a method enabling a probe to trace the surface of a sample, even if the surface is greatly "rough," by interlocking the movement of a fine adjustment mechanism and a coarse adjustment mechanism, based on the minute displacement state of the probe in an approaching/separating direction to the sample. A tunnel current detector detects a tunnel current, which starts to flow when a probe approaches the surface of the sample, within an atomic-level distance. This current is fed as an input signal to a fine adjustment mechanism control device, which in turn supplies an instruction signal to the fine adjustment mechanism. A device for detecting and evaluating the state of the fine adjustment mechanism receives positional data in the z-axis direction of the probe input from the fine adjustment mechanism control device, compares this data with reference data, and activates a course adjustment mechanism control device when it judges that the position of the probe cannot be changed as required only by a shift through the fine adjustment. When a course adjustment mechanism is to be driven, the course adjustment mechanism control device supplies an instruction signal, as necessary, the fine adjustment mechanism control device, so that a required adjustment to the probe position is executed.
While these examples of the prior art provide for the use of a pair of actuators to perform fine and course movements, they do not operate in a manner dividing the total motion into fast and slow components. What is needed is a method dividing the motion in this way, so that a broad range of slow movement can be allowed without compromising the resolution provided for fine movements.
The segmentation of tubular piezoelectric actuators, allowing the application of various driving signals to electrodes extending along various portions the surfaces of an individual actuator to obtain specific patterns of movement upon the application of driving signals among the electrodes. In examples described by Roland Wiesendanger in Scanning Probe Microscopy and Spectroscopy--Methods and Applications, Cambridge University Press, pp. 94-95, 1994, a piezoelectric tube actuator consists of a tubular section of piezoelectric material having an inner electrode extending along its inner surface and a segmented outer electrode extending along its outer surface. Each segment of the outer electrode extends longitudinally along its surface. As a voltage is applied to a single outside electrode, the tube bends away from that electrode. A voltage applied to the inner electrode causes a uniform elongation. For example, a small AC signal and a large DC offset can be applied separately to electrodes spaced 180-degrees apart.
Alternately the outer electrode is sectioned into four equal areas parallel to the axis of the tube. The tube is fixed at one end, and a probe is mounted at the opposite end to extend in a direction parallel with the axis of the tube. Pairs of opposite outer electrode segments are then excited by a voltage leading to bending modes of the tube, so that X- and Y-direction scanning motions of the probe occur.
Japanese Pat. Kokai No. 7-134133 also describes several versions of a tubular piezoelectric actuator having electrode segments extending along its outer surface. This actuator is also divided transversely into tubular sections having individual outer electrode segments.
While these references describe the construction of segmented tubular actuators, they do not address a method for causing these segments to respond respectively to a coarse portion of a driving signal and the remaining portion of the driving signal. Furthermore, this segmentation is applied to a tubular actuator. What is needed is the application of segmentation to a cantilever bimorph actuator, in which rapid movement can occur through simple bending, without the twisting distortion occurring in the walls of a tubular actuator.